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Before you can run a tap through a hole to cut threads, you’ll need to make the hole. Determining the correct hole size for the tap is important, and there are several factors to consider before just grabbing the nearest twist drill that happens to be a little smaller than the hole (bad idea!).
Good Tap Drill Information is Based on Thread Percentage
First thing is, where do you plan to get your information? The web is filled with tap drill size charts and many taps arrive with a recommended drill size right on the package. That’s one approach. A more comprehensive approach is to use a tap drill size calculator. As it turns out, there may not be one single size that is best for your tap, despite what the packaging or handy chart may say. The reason is that most of the time these resources don’t consider thread percentage.
What is a thread percentages?
Imagine the fully formed internal thread. Each thread rises from valley to peak. Now suppose you ran a twist drill down the hole and shaved off some of the peaks. They’re pretty delicate anyway and will wear off quickly. In fact, they contribute surprisingly little strength. Kennametal says a 100% thread is only 5% stronger than 75% thread. But here is the real kicker:
That 100% thread requires 3 times the power to tap!
Why do you care? because it is the power to tap that breaks taps, for one thing. Getting 95% of the strength with 1/3 the force on the tap means you’re dramatically less likely to break the tap off in the hole. Now we all know what happens when we break a tap, right? Sailors would go running out of the shop if they heard the language we use in that case. It’s just not a happy thing.
But is it okay to have less than 100% threads? In fact, many standards bodies insist on it. For example, American National and Unified thread specifications provide for a maximum of 83 1/3% thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread.
Before getting into how to select a thread percentage, let’s take a look at G-Wizard’s Tap Drill Calculator:
In this case, we’re looking at a 1/4-20 thread. The tap drills are listed in the table and sorted by thread percentage. Only standard twist drill sizes are shown. This makes it easy to scan down the list and pick the twist drill for the thread percentage you’re trying to achieve. Note that cutting taps and form taps have differing requirements, so you’ll want to be sure and pick the correct choice for your tap.
Guidelines for Selecting a Thread Percentage
- As mentioned, you can get 95% of the strength from a 75% thread, so when strength is the only consideration, this is the one I go for. Most charts and packaging recommendations are based on a 75% thread.
- Strength can be a function of length of threads. I don’t start thinking about maximizing strength unless I am limited by the fastening scenario. For example, if I can only have a very short threaded length that engages (perhaps due to threading a hole in a relatively thin plate), I will realize I need to higher thread percentage.
- OSG suggests 55-65% threads for most applications. The force required to drive a tap is way less than for 75%. This is the range I use most of the time unless I’m really worried about strength. If the length of the threaded hole is more than about 3x the diameter of the fastener, 55-65% threads are very likely going to be stronger than the fastener itself anyway.
- For very long thread lengths, minimize the percentage of thread as tapping forces in the bottom of the hole increase with the length of thread. For very long, read more than 5X the diameter of the fastener.
- For small diameter fasteners, consider using a lower thread percentage. Small taps are much more delicate and they’ll appreciate the lower forces required by lower thread percentages. I typically drop down to the 55% range for what seems to be a small tap.